Some effects have you choose a source of damage, often with a specific quality. The choice of a source is made when the spell or ability telling you to choose a source is resolving. For example, if you sacrifice a Burrenton Forge-Tender to activate its ability, you will choose a red source when this ability resolves.

A source of damage is any permanent in play, spell on the stack, an object referred to by a spell on the stack, a delayed triggered ability that is waiting to trigger, or a replacement effect that is waiting to be applied. The most common sources of damage chosen are permanents in play or spells on the stack.